In a rotary printing machine adapted for perfecting, a turning device is required to turn the sheet being printed after one side of the sheet has been printed so that the other side of the sheet can be printed. A turning device, for example, may comprise a turning drum and at least one sheet-guiding cylinder, which are arranged upstream (in the sheet running direction) between two printing units. A sheet turning device of this type is disclosed in German Patent DE-AS 2,354,418. In the printing machine disclosed therein, three drums are arranged between two printing units for sheet transfer. One of the three drums which is assigned to the downstream printing unit is designed as turning drum for turning sheets. Arranged upstream of the turning drum is a transfer drum which has air flow channels arranged axially on its circumference at a predefined interval. The flow channels are groove-like and are used to provide air suction. This arrangement is intended to serve the purpose of resting the sheet tautly but without tension on the transfer drum so that the sheet can be transferred in registry to the turning drum. The turning drum and the transfer drum are not provided with any device for guiding the sheet to avoid smearing of the trailing section of the sheet which is hanging free during the sheet turning phase.
German Patent DE-GM 6,949,816 discloses a sheet transfer cylinder on which a sheet is held by grippers at the beginning and end of the sheet. The sheet is held firmly on the transfer cylinder by air suction effected by negative air pressure on the circumference of the sheet transfer cylinder. The sheet is thus intended to rest flat on the cylinder and be pulled toward the end of the sheet to keep it smooth. The sheet transfer cylinder of this type is unsuitable for guiding a freely hanging printed sheet, because of its arrangement of grippers for gripping the trailing edge of the sheet.
A device for pressing a sheet flat on an impression cylinder is described in European Patent Document EP 0,306,684 B1. After a sheet passes the printing zone, it is guided on the impression cylinder by means of compressed air in the region between the printing zone and the transfer region of a downstream turning drum.
German Patent DE 3,411,029 C2 discloses a sheet guiding device which provides a continuous guiding surface. The sheet-guiding device has air nozzles connected to air flow channels. German Patent Document DE 4,140,763 A1 discloses a sheet guiding device which raises the leading edge of a sheet from the impression cylinder during the turning phase by means of compression air and guides the front part of the sheet on a guiding wiper.
The arrangements described above for sheet-guiding have the disadvantage that during the turning phase in which the sheet undergoes a reversal of movement, no effective means is provided to convey the printed sheet from the upstream sheet-guiding cylinder, such as an impression cylinder or a supply drum, to the turning drum without smearing the imprinted image on the sheet.